Duplex frequency control and monitoring system



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PATENT OFFICE DUPLEX FREQUENCY CONTROL AND MONITORING` SYSTEM William S.Halstead, Washington, D. C.

Application October 8, 1932, Serial No. 636,888

15 Claims. (Cl. Z50-36) (Filed but not issued under the act of March 3,

1883, as amended April 30, 1928; 370 0. 757) The invention isparticularly applicable to radio transmission systems in which apiezoelectric crystal oscillator is employed to control the frequency ofthe emitted wave, and will therefore be described hereinafter with thattype of radio frequency circuit in view; but it will be readilyunderstood that this particularization constitutes no limitation on theinvention, for other forms of oscillators, such as magnetostrictionoscillators may be substituted for the piezoelectric frequencycontrolling crystals and the invention may also be employed inconnection with other oscillating devices not directly associated withthe radio art.

In prior applications of the piezo-electric crystal oscillator as afrequency control means for radio transmitters the degree of frequencystability of the emitted wave has been limited largely by the inabilityof mercury-column thermostats, or other devices with delicatemovingelements responsive to temperature variations, through theirassociated relay-controlled heater circuits, to -maintain the piezocrystal at a constant operating temperature over extended periods oftime. Since very small changes in the temperature of the crystal areresponsible for appreciable and increasingly objectionable variations inthe frequency of the emitted signal, which ordinarily are not noticed byan operator unless suitable monitoring apparatus is employed, moreaccurate and reliable temperature control means which do not depend onmoving thermo-responsive elements are desirable.

It is a primary object of the present invention to provide highlyaccurate means for obtaining a high degree of frequency stability inoscillating systems, without the use of thermostats or devices employingmoving thermo-responsive elements.

It is a further object of this invention to provide means whereby thebeat frequency between two oscillators of opposite temperature-frequencycoefficients is employed as the frequency control means for bothoscillators, one of which is connected as the frequency-determiningportion of a radio transmitter.

Another object of this invention is to provide,

with the same electrical frequency-control means, visual and audibleindication of variations between the frequencies of the two oscillatingsystems.'

It is still another object of this invention to I provide additionalmeans whereby temperature control of a piezo-electric crystal oscillatormay bel accomplished without either thermostats or re ays.

According to the preferred embodiment of the 10 present invention, theseobjects are accomplished 'by the generation of a variableaudio-frequency beat signal whose variations in frequency are correlatedwith and dependent on variations in the joint temperature of twooscillating frequencydetermining elements of oppositetemperaturefrequency coefficients, such as X- and Y-cut quartz arrangedin the input circuits of two independent wave generators, one of whichis employed as the frequency-'determining portion of l0 a radiotransmitter. The beat signal, or control wave, is then employed toactuate frequency responsive and selective devices whose operationregulates the temperature of the two frequencydetermining elements andthereby controls their 28 respective frequencies.

In a selected application of this invention hereinafter described, onecrystal, preferably of negative temperature-frequency coicient, such asX-cut quartz, serves as the frequency-determining element of anoscillating circuit whose output is connected to the radio frequencyamplifier of a radio transmitter. A portion of the output from thecrystal oscillator circuit is supplied to a detector circuit whose inputalso carries'radio frequency energy from the output of an independentmonitoring crystal circuit whose frequency is determined by the secondcrystal. in this example of positive temperature-frequency coeflicientsuch as provided by Y-cut 40 quartz. Both crystals are jointly heated toa specified normal operating temperature at which the frequencydifference between the frequencies of their associated oscillatorcircuits produces in the common detector circuit an audio-frequency 45beat signal, the frequency of which decreases tol wards zero beat as thetemperature of the two crystals is increased beyond the specified normaloperating temperature. The beat signal, which is the result of thefrequency difference between the transmitter control circuit frequencyand that of the monitoring crystal circuit is then employed toaccomplish the following results: first, accurately regulate the degreeof heat applied to the two crystals and thereby maintain within 55 smalllimits the emitted frequency variations of aradio transmitter; second,provide indication of variations of frequency of the transmitter--control crystal oscillator from the monitoring oscillator frequency bymeans of a precise visual frequency indicating device, such as aresonant frequency meter, and an audible frequencychange indicator suchas a loud speaker connected to the input of the visualfrequency-indicating device.

The invention will be more fuily'understood from the following detaileddescription of a present proposed embodiment thereof: reference for thispurpose being made to the accompanying drawings, in which:

Figure 1 is a diagram of the frequency controlling portion of a radiotransmitter having piezoelectric frequency-temperature control andmonitoring means arranged inaccordance with the invention.

Figure 2 is an alternate arrangement wherein the electro-magneticrelay-controlled crystal heater of Figure 1 is replaced by aspace-discharge tube relay and associated crystal heater elements.

Figure 3 is a second alternate arrangement in which the resonantfrequency meter with its associated photo-electric relay-control meansand crystal heater as shown in Figure 1, is replaced by an electric wavefilter.

Figure 4 is a diagram illustrating the 4frequency variations of the twocrystal oscillators with changes in their temperature. Also, the diagramillustrates the corresponding beat frequency variation with identicalchanges in temperature.

Referring to the diagram of Figure 1, two piezo-crystals, I and 2, aremounted on lower electrodes 3 and 4, respectively, which are in turnmounted on a common perforated metallic plate or grille 5, connected toground. Protective covers, 6 and 1, enclose crystals I and 2respectively,

and serve as supports for upper electrodes 8 and 9, respectively.

Crystal I, the frequency-determining element of the transmitter properis preferably of X-cut quartz, or tourmaline, possessing a negativetemperature coefficient, i. e., its frequency of vibration decreaseswith an increase in temperature. Crystal 2, the monitoringfrequency-determining element is preferably Y-cut quartz in which thetemperature coefficient ispositive, its frequency increasing withincrease in temperature.

Upper electrodes 8 and 9, associated with crystals I and 2,respectively, are connected to the control elements of space dischargeoscillators I0 and II respectively. Sources of negative grid potentialI2 and I3, respectively, are connected between the grid and filamentcircuits of the two tubes III land II, respectively, throughradiofrequency choke coils I4 and I5, respectively, for supplying theproper bias potentials to the two grids. Sources of E. M. F. I6 and I1supply filament-heating current to the two oscillator tubes I0 and II,respectively. Space current is supplied to tubes I0 and II from directcurrent sources of E. M. F. I8 and I9, respectively, through the radiofrequency choke coils 20 and 2I, respectively, and plate inductances 22and 23,

respectively, Inductance 22, in the plate circuit of oscillator tube I0, and radio frequency by-pass condenser 24 connected between the plateinductance and ground,complete the output circuit elements of theoscillating circuit of which crystal I is the frequency-determiningportion.

Radio frequency energy is transferred from the output circuit of crystaloscillator tube I0 to the succeeding amplifier stages of the transmitterby coupling means such as condenser 25, connected between the plate endof inductance 22 and the 5 control grid of the first radio frequencybuifer" amplifierv 26.

Inductance 23, in the plate circuit of oscillator tube II, and radiofrequency by-pass condenser 21 connected between the plate inductanceand 10 ground, complete the output circuit elements of the monitoringcircuit of which crystal 2 is the frequency-determining portion.

Coupling means such as coils 28 and 23 are employed to transfer a smallamount of radio 15 frequency energy from plate inductance 22 to theinput circuit 30 of detector tube 3|.' Coupling means such as coils 32and 33 also transfer a small amount of radio frequency power from theplate inductance 23 of oscillator tube II to 20 the inputcircuit 30 ofdetector tube 3l.

Isolating or buffer amplifiers 34 and 25 are inserted in the couplingcircuits between the plate inductances 22 and 23, respectively, and thecommon grid circuit 30 of the detector tube 3| to prevent undesirableinteraction between the two output circuits of the oscillators.

The plate circuit 36 of detector tube 3l is connected, through theprimary windings of amplifying transformer 31 and source of platecurrent 30 38, to ground. A radio frequency by-pass condenser 39 isconnected between the plate of detector tube 3| and ground. Thesecondary windings of amplifying transformer 31 are connected to audiofrequency amplifier 40. The output tera6 minals of the amplier, shown at:zzx, are connected to a tuned frequency-responsive device, such as aresonant frequency meter 4I, provided with photoelectric means wherebythe movement of the pointer of the frequency meter beyond a specifiedpoint on the meter scale controls the operation of the photoelectriccircuit. A light-controlling medium, such as the curved refleetingelement 42, is fastened to the pointer arm 43, of the frequency meter.In the arrange- 45 ment shown in the diagram of 'Figure l, light fromasmall incandescent lamp 44 is focused, by means of lens system 45, onthe upper end of reflecting element 42 when the pointer arm is in thevertical position, as shown in Figure 1. The 50 curvature of thereflecting element 42 is such that movementof the reflector to the leftof the vertical position with increase in frequency above 500 cycleswill not interfere with the passage of the beam of light from the lamp44 through lens 55 systems 45 and 46 to a photoelectric cell 41.However, any movement of the reflector 42 to the right of the verticalposition with decrease in frequency below 500 cycles will immediatelyremove the reflecting surface from the incident light 30 beam, therebycutting off the light supply to the photoelectric cell. 'I'helight-sensitive cathode of photoelectric cell 41 is connected to thegrid of a vacuum tube 48. The anode of photoelectric cell 41 isconnected to the plate of vacuum tube 55 48, such connections producingan increase in current in the plate circuit of vacuum tube 48 with anincrease in light received by the photoelectric cell. A source of gridpotential 49. means for regulating the grid potential, such as poten- 70tiometer 50 shunting grid bias source 49, and a grid leak 5I connectedin series between the moveable arm of the grid potentiometer and thegrid, complete the elements of the input circuit of tube 48. Heatingcurrent for the cathode of 74 tube 48 is' supplied 'from a source of E.M. F. 52. 'Ihe plate current pathiincludesin series connection, a sourceof E. M. F. 58 and windings 54 of an electric relay 55. The contacts ofrelay arma'- 'ture 56 are connected in parallel with `resistor 51 andcondenser 58, which elements are conr nected in series with crystal.heater resistor 59,

power switch 60, and a source of heater power 6I. A motor-driven fan 62provides means for securing proper heat distribution throughout thecrystal oven. Audible indication of frequency change in the beat signalis provided by loud l speaker 11, connected to the input of amplifier40.

. perature-frequency coefficient is negative. Conversely, as thetemperature coefficient of monitoring crystal 2 is positive, itsfrequency increases directly with the temperature. With reference toFigure 4, assuming that transmittercontrol crystal I, of negativetemperature coefcient, is ground to a 2000 kilocycle fundamentalfrequency at a specified normal operating temperature of 50 degreescentigrade, shown at X, it will be seen that its frequency decreasestowards 2000 kilocycles from a higher frequency as the temperature isincreased. Monitoring crystal 2, of positive temperature coefficient,ground to a monitoring frequency 500 cycles below 2000 kilocycles at thespecified normal operating temperature of 50 degrees centigrade, on theother hand approaches its monitoring frequency, 1999.50 kilocycles, froma lower frequency as shown by the broken line. It may be seen by furtherreferencev to the diagram that if the temperature of both crystals werepermitted to rise several degrees above 50 degrees, the normal.operating temperature, the frequencies of both crystals would coincide.This point of zero beat is illustrated in the diagram as theintersection of the two dotted lines, representing the convergence ofthe two frequencies towards a common frequency with uncheckedtemperature rise--a condition which is not permitted to exist in theoperation of the circuits illustrated.

The radio-frequency coupling coils 28-29 and 32-33 connected between theoutput circuits of oscillator tubes I and Il respectively, and thecommon grid input circuit 30 of detector tube 3| thus supply to detectortube 3l two frequencies both of which, as the temperature increases,approach a common zero beat frequency from opposite directions. Theaudio beat frequency in the plate circuit of the detector tube,resulting from the interaction of the two radio frequencies scale, inwhich position the reflecting element 42, carried by pointer arm 43, aspreviously outlined reflects light from lamp 44 to the photoelectric`icell 41, thereby increasing the plate current of vacuum tube 48 which.action closes relay 55, 5

shunts resistor 51 and applies maximum heater current to the crystalheater element '59. The temperature of the crystals increases rapidlywith application of full heater power thereby accelerating the decreasein beat frequency. When 1o the beat frequency decreases slightly below500 cycles the position of pointer arm 43 is such that it removesreflecting element 42 from the incident light beam from lamp 44.Photoelectric cell 41 receives nolight, the plate current -in vacuum l'tube 48 decreases, relay 55 opens and the current in the crystal heaterelement 59 is reduced. As a result, the temperature of the crystalsdrops, the beat frequency increases and the indicator lpointer arm 43again moves reflector 42 into the incident 20 light beam, therebyclosing relay 55, through the intermediary action of va'cuum tube 48,which condition produces an increase in the heater current and adecrease in beat frequency. This cycle is repeated successively untilthe alternate application and reduction of full heater power soonmaintains the' average temperature and frequency within close limits.

It may be noted, by referring to Figure 4, that as the frequency of theoscillating circuit, deter- 3,0

vmined by X-cut crystal I, changes only 50 cycles,

while the beat frequency changes 200 cycles, with one degree variationin temperature, the emitted frequency of the transmitter does not varyappreciably. v.At the same time, any undue frequency variations in beatfrequency are visually and audibly indicated to the transmitterattendant. Failure vor improper operation of any element in the systemis indicated immediately by pronounced movement of the pointer of thefre- 1qiuency meter away from the normal vertical posi- In a modifiedform of the duplex crystal frequency-temperature control system,illustrated in Figure 2, the cathode of the photoelectric cell, 41, isconnected to the grid of a vacuum tube or a gas-content tube such as athyratron, 63, which is employed as an electronic relay in lieu ofmagnetic relay of Figure 1. The plate of vacuum tube or thyratron 63 isconnected through small 50 heater elements 64 and 65.,.arranged inseries and disposed directly beneath the lower electrodes of crystals Iand 2, respectively, to a source of alternating E. M. F. 66. Amilliammeter, 61 may be included in the plate circuit of tube 63 to give55 visual indication of the plate current. The anode of thephotoelectric cell 41 is connected to one en d of va secondary winding68 of transformer 69, the other end of secondary winding 68 beingconnected through a condenser 10 to the grid of ltube 60 63. Apotentiometer 1I is disposed across a section of secondary Winding 68 tovary the current flow through photoelectric cell 41. The mid-tap of thepotentiometer is connected to one side of the filament of tube 63,`which is also connected to one side of the primary Winding oftransformer 69. Current from transformer secondary 12 is employed toheat the cathode of tube 63. A power supply switch 13 is used to placethe sysm tem in operative condition. The connections, as described,produce an increase in current in the plate 4circuit of vacuum tube 63with an increase in the light received by the photoelectric cell. If

a thyratron is used, the circuit connections are 16 beam from exciting'lamp 44 through aperture 15 to photoelectric cell 41 when the pointeris to the right of the vertical position, corresponding to a decrease inbeat frequency below 500 cycles.

The principle of operation of the circuit of Figure 2 is similar'to thatof the circuit previously described and illustrated in Figure 1. "Themovement of the pointer of the frequency meter to the right or left ofthe vertical position with decrease or increase in frequency controlsthe operation of l the photoelectric cell so as to decrease or increase,

respectively, the plate current flowing through crystalheater elements64 and 65, thereby providing automatic temperature regulation of thecrystals without the use of a thermostat or an electro-magnetic relay.

In the arrangement of` Figure 3 a high-pass filter 16 is employed as asubstitute for the resonant frequency meter 4I with its associatedphotoelectric cell 41 and amplifier 48 of Figure 1 to control operationof heater relay 55. The highpass filter, which preferably is designed tocut off all frequencies below 500 cycles and pass without markedattenuation all frequencies above the- 500 cycle cut-off frequency, isconnected between the output as-x of the audio frequency amplifier 40,and the windings 54 of relay 55 of Figure 1.

In operation, the high-pass lter performs a function similar to that cfthe resonant frequency meter with its associated photoelectric circuitcontrol means, namely, it acts as a frequency selective device whichautomatically closes relay 55 thus applying full current to the crystalheater element'59 when the beat frequency is above 500 cycles, and opensthe relay, thereby reducing the current supply" to the heater elementwhen the beat frequency falls below 500 cycles.

It will be recognized that the illustrative systems described herein arecapable of considerable modification and rearrangement without departingfrom the spirit and scope of the invention, and it is therefore to beunderstood that Vthe following claims embrace all such modications andequivalent arrangements as may fairly be construed to fall within thescopeof the invention.

I claim:

1. In a frequency control and monitoring system employing oscillatingcircuits under control ofl frequency-determining resonators of oppositetemperature frequency coeillcients; common heating means for saidresonators; means for producing resultant-wave oscillations of variablefrequency, the frequency variations of said resultant wave beingdependent upon the temperature changes of said resonators; means foramplifying said resultant wave oscillations; and resonantfrequency-selective means for utilizing the frequency changes of theresultant wave to control the temperature of said resonators, andsimultaneously to provide precise visual indication of the degree offrequency change in said resultant wave, including a resonant frequencymeter having an indicating needle whose response to frequency variationscontrols the current flow through said common heating means therebyeffecting temperature frequency control of said resonators.

2. In a frequency control system employing oscillating circuits undercontrol of frequencydetermining piezo-electric crystals of oppositetemperature frequency coemcients; means for jointly heating saidcrystals; means for producing resultant wave oscillations of variablefrequency, the frequency variations of said resultant wave beingdependent upon the temperature changes of said crystals; means foramplifying said resultant wave oscillations; and resonant frequencyresponsive means for utilizing the frequency changes of the resultantwave to control the temperature of said crystals and simultaneously toprovide precise visual indication of small fre- -quency changes of saidresultant wave, including a resonant frequency meter; an indicatingneedle on said meter for visually indicating frequency variations incycles per second; a lightcontrolling element disposed on one extremityof said indicating needle; a light source; a photoof said crystals.

3. In a frequency control system employing oscillating circuits undercontrol of frequency-determining resonators of unlike frequencytemperature coeillclents; electric heating means for changing thetemperature of said resonators; means for producing resultant waveoscillations ofvariable frequency, thefrequency variations of saidresultant wave being dependent upon the temperature changes of saidresonators; means for amplifying said resultant wave oscillations;

and frequency selective means for utilizing the 4 frequency changes ofthe resultant wave to control the temperature changes of said resonatorsand simultaneously to provide precise visual indication, in terms ofcycles per second, of variations in the frequency of said ampliedresultant wave, includinga tuned electric filter; an electric relayconnected to the output circuit of said lter, the armature of said relaybeing connected in series with said electric heating means and a sourceof electric power; anda resonant frequency meter connected to the inputterminals of said electric lter.

4. In a frequenc control and monitoring system employing oscilla ingcircuits under control of frequency determining piezo-electric crystalsof unlike temperature frequency coefcients; an electric heating meansfor changing the temperature of said piezo-electric crystals; means forcombining the outputs of said oscillating circuits so as to produceresultant-wave oscillations of variable frequency, the frequencyvariations of the resultant wave being dependent upon the temperature ofsaid piezo-electric crystals; means for amplifying said resultant waveoscillations; and resonant frequency-selective means for utilizing thefrequency changes of said resultant wave to control the temperature ofsaid piezo-electric crystals, -and simultaneously to provide precisevisual indication of variations in the frequency of said amplifiedresultant wave. including a l posed on said indicating needle and inline with said light-aperture at a predetermined position of saidindicating needle corresponding to a specic frequencyvalue of saidresultant wave; and an electric relay whose controlling circuit isconnected to said photoelectric cell and whose controlled circuit isconnected in series with said electric heating means for regulating\ thetemperature of said piezo electric crystals.

5. In combination, two oscillating circuits, each controlled byfrequency-determining resonators of opposite frequency-temperaturecoemcients; electric heating means disposed so as to jointly change thetemperature of said resonators; a demodulator for combining the outputsof said oscillating circults to produce a resultant wave whose frequencyvariations correspond to the frequency difference between said twooscillating circuits; an amplifier for increasing the wave energy ofsaid resultantwave; a resonant frequency meter connected to the outputof said amplifier; an indicating needle on said frequency meter forvisually indicating small variations in the frequency of saidresultantwave; a source of light; a photoelectric cell; an aperturedisposed in said frequency meter between the source of light and thephotoelectric cell: a shutter disposed on said indicating needle, saidshutter being positioned in line with said aperture at a predeterminedposition of said needle, thereby controlling the passage of light raysfrom said source of light to said photoelectric cell in accordance withvarying positions of said needle; and a space discharge tube connectedto the output of said photoelectric cell; the output circuit of saidtube being connected in series with said electric heating means and asource of electric power, thereby utilizing the current flow throughsaid tube to vary the temperature of said resonators.

6. In combination, two oscillating circuits having frequency-determiningpiezo-electric resonators of unlike frequency-temperature coeflcients;electric heating means disposed so as to jointly change the temperatureof said resonators; a

demodulator circuit for combining the output of said two oscillatingcircuits to produce a resultant wave whose frequency variationscorrespond to the frequency difference between said oscillatingcircuits; an amplifier for increasing the wave energy of said resultantwave; a resonant frequency-meter connected to the output of saidamplifier; a movable needle on said frequency-meter for visuallyindicating small variations in the frequency of said resultant wave; asource of light disposed on one side of the face of said frequencymeter; a photoelectric cell disposed on the opposite side of the face ofsaid frequency meter; an aperture disposed in the face of saidfrequency-meter between the source of light and the photoelectric cell;a shutter disposed on said movable needle, said shutter being positionedin front of said aperture at a predetermined position of said needle; anelectron tube having its input circuit connected to the photoelecriccell; and an electric relay having its windings connected to the outputcircuit of said electron tube, said relay also having its armatureconnected in series with said electric heating means and a source ofelectric power foreflectins temperature control of'said piezo-electricresonators.

'7. In afrequency stabilizing and monitoring sytem, the combination oftwo oscillating circuits having frequency-determining resonators ofunlike temperature frequency coeflicients; electric heaters disposed soas to aiect the temperature of said resonators; a demodulatorcircuit forc ombining the output of said two oscillating circuits,

for producing a resultantwave whose frequency variations are thefrequency differences ofsaid two oscillating circuits; an amplifier forincreasing the wave energy of said resultant Wave; a resonantfrequency-meter actuated by said amplii'led resultant wave to effectvisual indication of frequency variations of said wave; a source oflight; a photoelectric cell positioned to respond to variations of lightintensity from said source, said variations being effected by themechanical actuation of said frequency-meter by the resultant wave; andelectrical means for translating the response of said photoele'ctric'cell into variations in current in said electric heaters.

8. In a frequency stabilizing and visual frequency monitoring system,the combination of two oscillating circuits having frequency-determiningpiezo-electric crystals of opposite temperature-frequency coeicients,one of said oscillating circuits having a piezo-electric crystal ofnegative temperature frequency coeicient, the other of said oscillatingcircuits having a piezoelectric crystal of positivetemperature-frequency coefficient; a radio frequency amplier connectedto the output circuit associated with said oscillator having apiezo-electric crystal of negative temperature-frequency coeflicient;said radio frequency amplifler being connected to the power tals; ademodulator f or combining the wave en ergy from said Wave generators inthe form of a resultant wave of varying frequency, said varyingfrequency representing the frequency difference between the waves ofsaid wave generators; an amplifier for i` creasing the wave energy ofsaid resultant wave; a resonant frequency-meter connected to the outputof said amplifler having a movable pointer-arm for visually indicatingfrequency changes of said resultant wave; a light source; aphotoelectric cell disposed in the path of light rays from said lightsource; a shutter disposed on said movable pointer-arm for eiiectingcontrol of the amount of light received from said light source by saidphotoelectric cell in accordance with variations in the frequency ofsaid resultant' wave; and an electron tube, the input circuit of saidtube being connected to said photoelectric cell, for amplifying thevariations in photoelectric current fiowing through said photoelectriccell, and the output circuit of said tube being connected in series withsaid electric heater means and a source of electric power for effectingchanges in the temperature of said resultant wave in accordance withchanges in the frequency of said resultant wave.

10. In a frequency stabilizing `and monitoring system, the combinationof two wave-generator circuits having frequency-determining crystals ofunlike temperature-frequency coefiicients; electric heater means foreiecting changes in the temperature of said frequency-determiningcrystals; a demodulator for combining the wave energy from said twowave-generator circuits in the form of a resultant .wave whose frequencyvariations are the frequency differences of said two wave-generatorcircuits; means for amplifysaid pointer-arm corresponding to denitefrequencies of said resultant wave; and an electronic amplifier whoseinput is connected to the output circuit of said photoelectric cell toeffect amplification of the current flow through said photoelectriccell, the output circuit of said electronic amplier being connected inseries with said electric heater coils and a source of electric powerfor effecting changes in the temperature of said resultant wave inaccordance with changes in the frequency of said resultant wave.

11. In a frequency control system employing oscillating circuits undercontrol of frequency determining elements of unliketemperature-frequency coefficients; means for producing resultant waveoscillations of variable frequency, the frequency variations of saidresultant wave being dependent upon the temperature changes of saidfrequency-determining elements; means for amplifying said resultant waveoscillations; and resonant frequency-selective means for utilizing thefrequency changes of said amplified resultant wave oscillations tocontrol the temperature of said frequency-determining elements,including a resonant frequency meterhaving a moving element 'whoseactuation visually indicates definite frequency variations in terms ofcycles per second; a source of light; a photoelectric cell; and meansactuated by said moving element for varying the amount of light receivedfrom said light source by said photoelectric cell in accordance withvariations in the frequency of said resultant wave oscillations, saidphotoelectric cell thereby effecting control of the temperature of saidelements.

l2. Means for producing a primary wave of substantially constantfrequency and simultaneously effecting positive visual indication c-fvariations in the frequency of said primary wave, comprising a primarywave generator whose oscillation frequency is controlled by apiezo-electric crystal having a negative temperature-frequencycoefficient; a second wave' generator whose oscillation frequency iscontrolled by a piezo-electric crystal having a positive temperaturefrequency coeflicient; electric heating means for varying thetemperature of said crystals; means for producing a resultant wave whosefrequency is the difference of the frequen- 6 cies of the waves of saidwave generators; means for amplifying said resultant wave; and resonantfrequency selective means for employing said amplified resultant wave tocontrol the temperature of said two piezo-electric crystals andsimultaneously to effect visual indication of frequency variations ofsaid resultant wave, including a resonant frequency meter having anindicating scale calibrated in cycles per second.

13. Means for producing an electric wave of 16' substantially. constantfrequency and simultaneously effecting positive visual indication ofvariations in the frequency of said electric wave, comprising two wavegenerators having frequency-determining elements whose tempera- 20.-

ture coefficients are of opposite sign; means for jointly heating saidfrequency-determining elements; and resonant frequency selective meanscontrolled by the difference of frequency of said wave generators toregulate the degree of heat 2l.;

influencing said frequency-.determining ele-3 ments, comprising anelectric wave filter, and an electric relay selectively operated by saidfil ter, said relay being arranged to control the temperature of saidfrequency-determining elements. 3.

14. I n a frequency control system employing oscillating elements whoserespective frequencies of oscillation are under temperature control;heterodyne means for producing a resultant control wave whose frequencyvariations are-85 dependent upon temperature variationsof saidoscillating elements; heating means for varying the temperature of saidoscillating elements; and photoelectric means for limiting frequencyvariations of said control wave, including a res- 40- onant frequencymeter having a movable frequency responsive member whose displacementcontrols the amount of light received lby said photoelectric means, saidfrequency meter and said photoelectric means being operativelycoordinated with said resultant control wave and said heating means.

15. In a frequency control and monitoring system for stabilization ofthe emitted wave of a radio transmitte two wave generators having 50.

frequency-determining vibratile resonators of unliketemperature-frequency coefficients; a common source of heat for varyingthe temperature of said resonators; a demodulator for combining thewaves from said generators in the form of a resultant wave representingthe frequency difference between said waves from said generators; anamplifier for increasing the wave energy of said resultant wave; andresonant frequency selective means connected to the output of saidamplifier for effecting control of the temperature of said vibratileresonators, including an electric wave filter whose input circuitiscon-y nected to the output of said amplifier and whose output circuit isconnected to an electric relay. the operation of said` relay effectingtemperature control of said vibratile resonators.

WILLIAM S. HAISTEAD.

